The present application claims priority to Japanese Application No. P10-234657 filed Aug. 20, 1998, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a solid electrolyte battery and, more particularly, to a lithium secondary battery or a lithium ion secondary battery exploiting the solid electrolyte. More particularly, it relates to a solid electrolyte battery superior in the electrode utilization factor, cyclic characteristics and in load characteristics and which can be increased in capacity.
2. Description of the Related Art
The lithium ion secondary battery is loaded on a wide variety of electronic equipments because of its high energy density. However, with the conventional lithium ion secondary battery, employing a combustible organic solvent, security against e.g., liquid leakage in case the battery is destroyed or fractured by some reason or other is not sufficient. In light of this, a solid electrolyte battery employing a solid electrolyte is stirring up notice.
In producing a solid electrolyte battery in general, high molecular organic compounds, exhibiting lithium ion conductivity, are coated on both of positive and negative electrodes, and the resulting two electrodes are stacked together. Alternatively, a solid electrolyte film is sandwiched between positive and negative electrodes to manufacture the battery. If the solid electrolyte is an organic compound having polymerizable functional groups, thermal or light polymerization steps are occasionally added in the battery manufacturing process.
This solid electrolyte battery is superior in safety and in energy density, while it can be reduced in size and thickness.
However, in this solid electrolyte battery, the following performance as the battery is not desirable.
That is, in the solid electrolyte battery, since both the electrolyte and the electrodes are solid, electro-chemical bonding between the particles of the active material of the electrodes and the solid electrolyte film on the electrode interface is inferior such that the interface impedance is significantly higher than with the battery of the electrolyte solution type to produce significant polarization, as a result of which the battery performance is significantly lowered.
As means for solving this problem, it has been proposed to use a compound electrode prepared on mixing a solid electrolyte to an electrode composition to increase the contact between the solid electrolyte and particles of the active electrode material.
However, with the method of employing the compound electrode, since many solid electrolytes are poor in their functions as the binder, such that it is difficult to realize a sufficient bonding strength between the current collector and the electrode composition. The result is that the layer of the electrode composition is collapsed, or the layer of the electrode composition is separated from the collector, due to expansion and contraction of the particles of the active electrode material consequent upon repeated charging/discharging, thus deteriorating cyclic characteristics. In order to prevent this phenomenon from occurrence, electrically conductive assistant agents need to be dispersed, as a result of which the battery capacity is lowered. Alternatively, complex processing, such as cross-linking, for improving the strength of the solid electrolyte used as a binder, is necessitated.
It is therefore an object of the present invention to provide a solid electrolyte battery in which contact between the electrolyte and the electrode is created to improve battery characteristics significantly.
In one aspect, the present invention provides a solid electrolyte battery having a positive electrode, a solid electrolyte and a negative electrode, wherein the positive electrode and/or the negative electrode is made up of a current collector and a layer of an electrode composition mainly composed of a binder and powders of an active electrode material. The electrode composition layer is layered on the current collector and has a mean layer thickness within xc2x110% of the mean particle size of the powders of an active electrode material.
In another aspect, the present invention provides a solid electrolyte battery having a positive electrode, a solid electrolyte and a negative electrode, wherein the positive electrode and/or the negative electrode is made up of a current collector and a layer of an electrode composition mainly composed of a binder and powders of an active electrode material. The powders of the electrode active material is adapted to be contacted with the current collector. The electrode composition layer is layered on the current collector and has a mean layer thickness within xc2x110% of the mean particle size of the powders of an active electrode material.
In the solid electrolyte battery according to the present invention, as described above in detail, the layer thickness of the layer of the electrode composition is set so as to be within a pre-set range with respect to the mean particle size, whereby the layer thickness of the electrode active material is approximately equal to each other, on an average, such that it is possible to charge the particles of the electrode active material evenly in the layer of the electrode composition without overlapping along the direction of thickness. Thus, the totality of the particles of the electrode active material are contacted with the solid electrolyte or the current collector to provide an optimum interface between the electrode active material and the solid electrolyte.
Moreover, in the solid electrolytic battery according to the present invention, since the particles of the electrode active material are formed to contact with the current collector to form a solid electrolytic between the two electrodes, both electron conduction paths and lithium ion conducting paths are formed optimally on the entire particles of the electrode active material.
Therefore, the solid electrolytic battery according to the present invention is a high-capacity solid electrolytic secondary battery having superior load characteristics and cyclic characteristics.